HVAC systems can be used to regulate an environment within an enclosure. Typically, a circulating fan is used to pull air from the enclosure into the HVAC system through ducts and push the air back into the enclosure through additional ducts after conditioning the air (e.g., heating or cooling the air). For example, a gas furnace, such as a residential gas furnace, is used in a heating system to heat the air.
Flame rectification to sense presence or absence of a flame is conventional in gas furnace controls technology. Typically, a 120 volt AC power is coupled to a flame-probe through a first capacitor. When no flame is present, a second capacitor coupled to the flame-probe is charged to a selected value of, for example, 5 volts DC, through a resistor connected to a DC voltage source. A change of state device, such as an inverter, has an output connected to a microprocessor and an input connected to the second capacitor. When no flame is present, the second capacitor maintains the voltage at an input of the inverter above a threshold value so that an output of the inverter is low, thereby providing an indication to the microprocessor that there is no flame. When a flame is present, the second capacitor discharges to ground through the flame which acts as a poor diode connected in series with a resistor. When the second capacitor discharges to a level below the threshold, the inverter changes state with its output going high thereby providing an indication to the microprocessor that a flame is present.